
function TreeNode(val, left, right) {
    this.val = (val===undefined ? 0 : val)
    this.left = (left===undefined ? null : left)
    this.right = (right===undefined ? null : right)
}
function createTree(arr) {
  if (arr.length < 1)
    return null;
  let NodeArr = [];
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] == null) {
      NodeArr.push(null);
      continue;
    }
    let tempNode = new TreeNode(arr[i], null, null);
    NodeArr.push(tempNode);
  }
  for (let i = 0; i < arr.length; i++) {
    if (NodeArr[i] == null) continue;
    if (2 * i + 1 < arr.length)
      NodeArr[i].left = NodeArr[2 * i + 1];
    if (2 * i + 2 < arr.length)
      NodeArr[i].right = NodeArr[2 * i + 2];
  }
  return NodeArr[0];
}
function preShow(node) {
  if (node == null)
    return;
  console.log(node.val);
  preShow(node.left);
  preShow(node.right);
}
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var preorderTraversal = function(root) {
  let stack = [root], result = [];
  while(stack.length) {
    const node = stack[stack.length - 1];
    stack.splice(stack.length - 1, 1);
    if(node == null) {
      continue;
    }
    result.push(node.val);
    stack.push(node.right);
    stack.push(node.left);
  }
  return result;
};
let root = [1,null,2,3];
console.log(preorderTraversal(root));
root = [];
console.log(preorderTraversal(root));
root = [1];
console.log(preorderTraversal(root));
root = [1,2];
console.log(preorderTraversal(root));
root = [1,null,2];
console.log(preorderTraversal(root));
